A master clutch functions as a releasable coupling between engine and transmission components in a motor vehicle. A typical clutch consists of driving discs which are forced together by a spring to squeeze a driven disc located between them. The driven disc is typically lined with friction material. The driving discs are attached to the engine output while the driven disc is attached to the transmission input shaft. Most applications utilize actuating levers which apply an axial force to the rotating discs to engage the clutch. Many actuating levers require a large range of motion for a comparatively small amount of disc motion to deliver a sufficient force to the clutch. For example, a typical ratio of lever motion to disc motion is 60:1 in many foot-actuated clutch installations.
When the clutch is being engaged, the actuator levers allow the discs to contact each other and, depending on the net interface force, a coupling torque is generated. The driving and driven discs slip against each other until the coupling torque is sufficient to couple the discs such that they rotate at substantially the same speed. This slipping condition occurs frequently during normal operation of a vehicle, such as when initiating vehicle motion.
As is well known, slipping the clutch wears the engaging surfaces resulting in a reduction of the steady-state thickness of the discs. The slipping also creates heat which causes a transient expansion of the clutch discs and friction material. Therefore, on subsequent clutch engagements, the distance the driving disc must travel toward the driven disc to generate the same force and resultant coupling torque varies depending on the temperature and wear state. Due to the magnitude of the travel ratio, what may appear to be a small amount of wear or heat expansion will substantially alter the force, travel, and position characteristics of the actuator.
It is desirable for the input mechanism to have a substantially constant force versus position relationship so that an operator can consistently control the clutch throughout the usable life of the clutch, regardless of the type of operator. For example, a clutch may be controlled by a human operator, or by a robotically or mechanically operated mechanism, either of which may employ a traditional foot pedal lever or a servomotor shaft connection.
Consistent clutch operation, then, necessitates adjusting the position of some part of the control actuator as the friction interfaces wear. Several kinds of adjustment mechanisms have been adopted to provide this functionality. They include devices with threaded rods to manually modify the effective lengths of a mechanical clutch linkage as well as automatic or self-adaptable column lengths of hydraulic fluid between the pistons of hydraulic actuation systems. The various adjustment systems have advantages and disadvantages. For example, a threaded rod adjuster is relatively simple and inexpensive but is often located near the rotating members of the clutch. Thus, these devices are difficult to adjust due to their poor accessibility in the clutch assembly.
Certain transmission systems, generally known as layshaft systems, impose additional control constraints upon the clutch actuator apparatus. These transmissions provide for multiple gear ratios by selectively engaging and disengaging gear teeth or dog clutch teeth. Preferably for these transmissions, the teeth to be engaged are rotating at substantially the same mating speed. This allows the edges of the teeth to physically engage each other while avoiding breakage, wear, and wear products attendant to raking collisions among the mating teeth.
When the transmission is in a neutral gear with the engine running and the master clutch is subsequently disengaged, the driven members tend to remain rotating due to their inertia. This inertia is relatively large in heavy-duty powertrain applications, such as in class 7 and class 8 trucks, since these vehicles contain large, massive components to accommodate their high torque demands. Additionally, clutch drag torque often causes the transmission input shaft to rotate at, or near, engine speed. This drag torque is due to spurious contact between the driving and driven discs and, in heavy-duty applications, is frequently 10 foot-pounds or more.
The residual rotation after the clutch is disengaged prolongs the time required to complete a gear shift in the transmission. This time may be significant in a heavily loaded vehicle ascending an incline. Under these conditions the loss in vehicle speed, while waiting for the input to slow down to the proper engagement speed for the next ratio, may preclude the desirability to complete the shift. The time period may also have a cumulative effect in that heavy-duty applications frequently require as many as fifteen shifts before reaching highway speeds. Therefore, it is desirable to provide a means to control the speed of the driven members after the clutch is fully disengaged.
To provide this control, layshaft transmissions are usually equipped with specialized brakes that overcome the residual rotation related to clutch drag and/or inertia torque. Some applications, such as the Roadranger.RTM. or Twinsplitter.RTM. transmissions, manufactured by the assignee of the present invention, include brakes which are removable, serviceable devices actuated interactively with clutch disengagement. The devices are generically referred to as clutch brakes, inertia brakes, or upshift brakes.
The robustness of a clutch brake varies depending upon its intended application. For light-duty applications, such as for use in engaging a starting gear without raking the dog clutches, a simple plastic or bronze piston, forced into the periphery of a primary drive gear, is sufficient. For more severe applications, such as the upshift brake incorporated into a Twinsplitter.RTM. transmission, a multiple disc brake is utilized to accommodate high torque and the resulting thermal capacity necessary for proper operation. This type of upshift brake is typically actuated by fluid pressure, such as hydraulic or pneumatic pressure, and is connected to the input shaft through a primary drive gear of the transmission.
The operator indicates a desire to utilize the upshift brake by extending the clutch actuator travel beyond a detent position. The detent position corresponds to the point of complete disengagement of the master clutch. The fully extended clutch actuator may activate a switch, a control valve, and a pressure source which is used to force the multiple friction plates of the upshift brake together (or the piston against the periphery of a primary gear), thereby applying the brake to slow or stop the rotation.
It is important to coordinate actuation of the upshift brake with the complete disengagement of the master clutch. If the upshift brake is applied prior to complete disengagement, the torque being transmitted through the master clutch may cause excessive heating and wear of the upshift brake resulting in premature failure. A delay in actuation of the upshift brake after the master clutch is fully disengaged is undesirable since it defeats the purpose of using a brake in the first place. Moreover, it makes the object of consistent control more difficult for an operator to achieve.
As is well known in the art, coordinated control of the master clutch and the upshift brake can be accomplished by manually coupling the actuators for these mechanisms. However, the change in a master clutch or upshift brake resulting from wear is accompanied by a change in the positional relationship between disengagement of the master clutch and application of the upshift brake. Since brake wear and clutch wear normally occur at different times and rates, and may even proceed in opposite directions, there is no consistent relationship between brake wear and clutch wear. Therefore, it is desirable to maintain a constant relationship between the disengagement position of the clutch and the apply position of the brake for consistent brake application. This requires adjustment of the brake actuator, the clutch actuator, or both.